The progress of printing technology, including digital printing, has made it possible to make very high quality prints on films typically of PVC. A widely used application of high quality printing, obtainable with modern techniques, consists of imitation of natural materials like wood.
For example, it is known to make laminates made up of a substrate coated with the printed film that reproduces the appearance of a precious material and/or a natural material. One of the most common applications is represented by the production of plastic films for floors, imitating wood, known as “imitation wood” or “imitation parquet” or simply “parquet laminate”. Other applications provide the use of printed films to add a touch of class to the surface of MDF panels and polylaminated plywoods for furniture surfaces, or coating of metal boards instead of painting, for the making of surfaces of household appliances and internal furnishing of boats.
Printing, albeit of high quality, is however not sufficient to give the product an acceptable appearance, because the film has a shiny and smooth surface which is absolutely unnatural. For example, it is possible to obtain a print that accurately reproduces the colours, the tones and the grains of any type of wood, but the product clearly remains perceptibly artificial due to the smooth and shiny surface.
In order to overcome this problem, it is known to use an embossing process. The embossing process generates a series of recesses on the surface of the film or laminate, creating irregularities that are perceptible to the touch, which allow the product to come much closer to the desired natural appearance.
A known technique applies the embossing in a continuous manner, substantially making the film or laminate pass between an engraved cylinder and an opposite pressing cylinder, also known as “offset cylinder” or counter-cylinder. This technique is quick, and substantially it can be implemented directly after the printing and the coupling between substrate and printed film. However, it is not satisfactory since the engraved cylinder generates an embossing with a pattern that is periodically repeated after every complete rotation of the cylinder, and that is not coordinated with the printing. For example, the distribution of the irregularities does not match with grains, knots or other special features of wood that are reproduced by the printing. Basically, the finished product remains clearly perceptible as artificial.
In order to avoid also this drawback, processes for registered embossing are known in which the irregularities produced by embossing are specifically coordinated with the printed decoration. As can be understood, the embossing member in this case is specific for a given decoration.
The process for registered embossing makes it possible to obtain a product that more realistically imitates a natural material (wood, stone, etc. . . . ), but it poses some technical difficulties. Indeed, it is necessary to ensure that the embossing is carried out strictly in a registered manner, so that the three-dimensional profile corresponds exactly with the print on the film. A possible misalignment or delay/anticipation of the engraved cylinder would render the whole process pointless creating a product with an unnatural and unsatisfactory appearance.
In the prior art, the registered embossing is carried out discontinuously. For example, a known technique for registered embossing a laminate comprises the following steps: the laminate is cut into panels of predetermined size, for example 2×4 meters; each panel is then conveyed individually to an embossing press; the panel is aligned on two axes, with respect to the plane of the embossing press and with the help of suitable references generated during printing; the press applies the desired relief (embossing) to the surface of the panel.
Production with discontinuous embossing does, however, prove to be slow and costly. The off-line embossing press, in particular, is expensive and bulky. A printed film or a laminate comprising a printed film, like the above mentioned “imitation parquet” laminates, are low-cost products and hence there is a need to simplify the process and make it as cheap as possible.
Due to the above, there is a need to operate the registered embossing of films or laminates continuously, i.e. directly on the sheet instead of on pre-cut panels.
Embodiments of the invention focus, in particular, on processes that use an extensible printed film, alone or in a laminate product comprising a support substrate coupled with the film. The term “extensible film” means a film that can undergo a substantial deformation. For example, films not reinforced with fibreglass are considered extensible, unlike reinforced films which are substantially inextensible.
The use of an extensible printed film poses a technical problem that up to now has not been solved and that has prevented the implementation of continuous synchronized embossing.
An extensible film, indeed, is subject to substantial changes in length that cannot be determined a priori, both during the printing process, and during storage or transportation before coupling and embossing. For example, the most common causes comprise:                excessive traction (also called “pull”) during the winding after the printing;        the storage time and/or temperature, which may allow tensions to develop inside the material, generated during the steps prior to embossing;        excessive traction and/or heating of the material during the unwinding, the possible coupling with a substrate and before embossing.        
A consequence of an elongation of the extensible film is that the pattern itself is deformed. A particularly important consequence is the alteration of the distance between reference marks printed on the film. In fact, it is a known technique to generate, during printing, a series of reference marks (also known as notches or markings) that are spaced by a defined and constant distance, equal to the circumference of the printing cylinders. Upon optical reading, said reference marks act as a reference for registered printing. However, the deformation of the printed film, for example an elongation due to excessive pull and/or heating, inevitably alters the distance between said marks. It is thus clear that said marks cannot be of help in controlling a registered embossing process, being affected by an error that is not known a priori.
In other words, use of an extensible film makes even more complicated to maintain the match between embossing and printing, due to the deformability of the printing support (i.e. the film). For these reasons, continuous registered embossing has up to now been considered inapplicable to processes that involve an extensible film. However, systematically using an inextensible reinforced film involves problems of higher cost and therefore there is interest in attempting to extend synchronized continuous embossing also to extensible supports.